This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Cobalamins are required cofactors in a number of enzyme reactions, including methyl cobalamin-dependent methionine synthase and adensoyl-cobalamin-dependent methyl malonyl-CoA mutase. These enzymes are inhibited by nitric oxide (NO), and it is believed that this is due to the formation of an NO-cobalamin complex. In this proposal, combined XAS, protein crystallography and time-dependent DFT calculations will be used to investigate the geometric and electronic structure of cobalamin-NO relative to that of aqua and methyl cobalamins.